1. Field of the Invention
The present invention relates to a method of diffusion brazing parts made of a nickel-, cobalt- or iron-based superalloy having a polycrystalline or monocrystalline structure. The method is particularly applicable to the joining of parts and to the repair of parts by resurfacing, particularly in the aeronautical field.
2. Summary of the Prior Art
The severe operating conditions imposed, for example, on the blades of turbo machines and of industrial turbines demand, in particular, excellent high temperature resistance to oxidation and/or excellent corrosion resistance combined with good high temperature mechanical properties such as good creep resistance, and have given rise to the use of nickel-, cobalt- or iron-based superalloys to fabricate them. For producing assemblies or carrying out repairs by resurfacing, the current techniques of welding with fusion prove to be poorly adapted to these materials or would be complex and often impracticable to carry out.
Diffusion brazing methods have therefore been used. These methods aim to obtain bonds which are homogeneous from the chemical and structural point of view, while avoiding deeply altering the metallurgical structure of the material. An example of such a method is disclosed in EP-A-0075497.
It is known, in order to carry out repairs by resurfacing, to use brazing powders known as dual-component powders and consisting of a mixture of two powders obtained, for example, by pulverizing under argon:
a superalloy powder with a chemical composition close to that of the material to be repaired; and PA1 a nickel- or cobalt-based powder containing 2 to 6% by weight of flux elements such as boron or silicon. PA1 the operation of mixing the two powders is lengthy and tedious, PA1 the homogenization of the chemical composition of the mixture is intricate and requires numerous checks, but remains vital, PA1 special storage conditions have to be provided in order to avoid any segregation of the mixture, and PA1 the ratio of the proportions of the two powders in the mixture should moreover be adapted according to the nature of the resurfacing and, in particular, to the width of the cracks to be repaired. PA1 (a) preparing a predetermined quantity of a superalloy powder having a composition identical to said superalloy A of said parts, and a quantity, in a weight ratio to said superalloy powder of between 0.1% and 40%, of a powder of at least one flux element selected from the group of boron and silicon; PA1 (b) mixing the powders prepared in step (a) and placing the powder mixture together with a plurality of balls into a container, then closing the container and maintaining it under a controlled atmosphere, said balls and said container being made of said superalloy A; PA1 (c) placing said container in a grinder and maintaining movement of the container for a period of from 30 seconds to 500 hours in order to incorporate the flux element onto the surface of the grains of said superalloy powder to thereby form a so-called single component powder; PA1 (d) chemically cleaning the surfaces of said parts to be brazed in a bath; PA1 (e) thermochemically treating said parts in a furnace; PA1 (f) depositing a filler on said surfaces of said parts, said filler consisting of said single-component powder obtained in step (c); PA1 (g) carrying out a thermal brazing cycle in said furnace wherein the temperature is increased in steps up to a brazing temperature of between 1050.degree. C. and 1400.degree. C. depending on the superalloy, and is then held for a time of from 15 to 30 minutes; and PA1 (h) carrying out a thermal diffusion treatment in said furnace under a controlled atmosphere for a period of between 4 and 16 hours.
The diffusion brazing operation is carried out at a temperature which lies between 1050.degree. C. and 1220.degree. C. and which is below the melting temperature of the superalloy. At the selected temperature of operation, the superalloy powder remains in the solid state and the powder containing the flux elements becomes liquid, thus ensuring the fluidity of the mixture. The brazing of the grains of superalloy powder together causes a densification and, by diffusion of the flux elements, isothermal solidification of the liquid joint is obtained at the brazing temperature. Interdiffusion between the brazed joint and the superalloy of the basic metal ensures the homogenization of the chemical compositions. After the conventional thermal treatments, the regions resurfaced by diffusion brazing exhibit the high-temperature mechanical properties sought, being very close to those of the basic material of the parts. Moreover, the known coatings for enhancing resistance to corrosion and to oxidation at high temperature may be applied.
However, the known diffusion brazing methods using dual-component powders entail certain constraints and drawbacks as regards implementation: